The present invention relates to a synchronous machine and to a process of manufacturing a synchronous machine and, more particularly, to a synchronous machine with permanent magnets on the rotor and cage for direct start.
Synchronous machines with permanent magnets are known from the prior art and comprise essentially a stator and a rotor, the rotor being substantially cylindrical in shape, having a surface portion and a rotor nucleus, and the rotor may be provided with a cage positioned on the surface portion, the cage being formed by parallel bars connected at their ends by short-circuit rings. Such a rotor configuration is known as a cage rotor and is useful in the process of starting the machine.
Inside the rotor, one may allocate pairs of permanent magnets in fixation grooves that are positioned longitudinally in the rotor nucleus (or rotor core).
As far as the electric characteristics are concerned, the rotor has a plurality of poles, which varies according to the characteristics and applications of each machine.
The permanent magnets, allocated inside the rotor, have the purpose of generating a machine-magnetization flux.
A common fact found on machines of this type is the flux leak, caused by the magnetic short-circuit of the magnets through the steel bridges formed at the following points: 1) between the grooves for allocation of the magnets of a same pole; 2) between the grooves for allocation of the magnets and the adjacent grooves of the cage 3) between the beak of the rotor grooves and the outer diameter of the rotor. This effect, as well as the flux-loss points can be better understood from FIG. 6, which shows the magnetic flux lines.
In this regard, it would be ideal be to provide the nucleus totally cut longitudinally with magnets, as can be seen in FIG. 9, since in this way there would be no loss of magnetic flux. The situation is only hypothetical, since in this case the rotor would not have the mechanical stability required for the functioning of electric machine.
One of the solutions of the prior art that discloses the use of the application of permanent magnets is described in document U.S. Pat. No. 6,876,119. According to the teachings of this prior art, one describes a synchronous motor having a rotor provided with V-shaped magnets, through the junction of magnets set to each other. Although this solution is good from the magnetic point of view, it may present problems relating to the mechanical stability (rigidity) of the blade packet, due to the large amount of steel removed during the stamping process. This fact, allied with the fact that significant deformations occur on the steel packet during the injection of aluminum, may cause problems of collision of the rotor, which impairs the reliability of the machine.
Such problems require subsequent treatment of the rotor for reducing the collision, such as grinding, machining or a similar process. The same reference further describes the possibility of configuring the grooves close to the end of the magnets with a greater depth, so that they will approach the magnet and prevent a short-circuit of the magnetic flux from a pole to the other on the same magnet.
This solution minimizes the problems of short-circuit of magnetic flux, but makes the construction of the electric machine difficult, since a cage-rotor groove needs to be especially configured and mounted in an specific manner during the process of manufacturing the machine, requiring differentiated stamping tools, which results in complications of practical and economical nature.
Another characteristic of this reference is the fact that is presents similar reluctances on the direct axis and on the quadrature axis. In this way, however, it is not possible to take advantage of the reluctance torques in the working condition, and this occurs for the following reasons:
The torque generated by a synchronous motor can be divided into two components:                (a) synchronous torque: it represents the synchronization torque between the magnet filed and the main spinning filed. It can be calculated by the following formula:        
                              T          sinc                =                              p                          2              ·              π              ·              f                                ·                      (                                                            V                  1                                ·                                  E                  f                                                            X                sd                                      )                    ·                      sin            ⁡                          (              δ              )                                                          Eq        .                                  ⁢        1            wherein:p=number of polesf=frequency [Hz]V1=feed voltage [V]Ef=EMF induced by the magnets [V]Xsd=synchronous reactance of direct axis [Ω]δ=load angle    a) Reluctance torque: it appears due to the difference in reluctance between the direct axis and the quadrature axis. It can be calculated by the following formula:
                              T          rel                =                                            p              ·                              V                1                2                                                    4              ·              π              ·              f                                ·                      (                                          1                                  X                  sq                                            -                              1                                  X                  sd                                                      )                    ·                      sin            ⁡                          (                              2                ·                δ                            )                                                          Eq        .                                  ⁢        2            wherein:Xsd=synchronous quadrature reactance [Ω].
The total torque generated by the motor in synchronous speed is the sum of Tsync+Trel. In this way, one can observe that the direct-axis reluctance and quadrature-axis reluctance have a great influence on the maximum torque value obtained.
More specifically, the influence of each part of torque and its variation as a function of the motor load angle can be better understood if analyzed graphically, as illustrated in FIG. 1. As shown by the equations, different values of direct axis reluctance and quadrature reluctance may lead to different values of maximum torque generated by the motor.
Taking as a basis a 2-pole motor, one can calculate the magnitude of the maximum torque for each of the configurations below, in FIG. 2a, a situation where the magnitude of the direct axis is substantially equal to that of the quadrature axis (Xd≈Xq); in FIG. 2b one illustrates a situation where the magnitude of the direct axis is smaller than that of the quadrature axis (Xd<Xq), and in FIG. 2c one illustrates a construction where the magnitude of the direct axis is greater than that of the quadrature axis (Xd>Xq).
For each of the alternatives, the Xd and Xq values were obtained by analysis of finite elements. By using the equations 1 and 2, one calculates the value of the total torque as a function of the motor load angle, as shown in the graph of FIG. 3. As can be seen in the graph, the maximum torque is greater for the case in which Xd<Xq. Most of the gain is due to the drop in direct-axis synchronous reactance, which increases significantly the value of the loss of the part of synchronous torque.